Determination of zinc oxide in sunscreen using ion chromatography with visible absorbance detection

Significance of the Topic

Determination of zinc oxide in sunscreen is essential for ensuring product safety and efficacy, as zinc oxide acts as a broad-spectrum UV blocker and its content must comply with regulatory limits. Modernizing compendial assays from manual titration to automated ion chromatography (IC) supports higher throughput, improved specificity, and alignment with USP monograph updates.

Objectives and Study Overview

This study aimed to develop and validate an IC method for quantifying zinc oxide in various sunscreen formulations, following the proposed USP General Chapter <591> Zinc Determination. The method replaces traditional titration in zinc-containing monographs and addresses challenges in sample preparation, particularly for water-resistant products.

Methodology and Instrumentation

• Instrumentation
  • Dionex ICS-5000+ HPIC system (Dual Pump, Column Heater, Degasser, UV/Vis Detector)
  • Dionex Chromeleon 7.2.5 Chromatography Workstation
  • Dionex IonPac CS5A analytical column with CG5A guard column
• Chromatographic Conditions
  • Eluent: 7.0 mM pyridine-2,6-dicarboxylic acid (PDCA), 66.0 mM KOH, 5.6 mM K₂SO₄, 74.0 mM formic acid
  • Flow rate: 0.30 mL/min; Column temperature: 30 °C; Injection: 2.5 µL
  • Post-column reagent: 0.5 mM PAR in buffered diluent; Flow rate: 0.15 mL/min; Detection: 530 nm
• Sample Preparation
  • Method A: Dissolution in 6 N HCl for non-water-resistant sunscreens
  • Method B: Addition of acetonitrile prior to 6 N HCl extraction for water-resistant formulations
• Validation Criteria
  • Specificity, linearity (0.1–30 µg/mL), LOD (0.1 µg/mL), LOQ (0.3 µg/mL)
  • Accuracy, precision, and robustness per USP <1225> and <621>

Main Results and Discussion

The IC method provided clear separation of Zn-PDCA complex at ~7.9 min, distinct from other transition metals. Calibration across 11 levels yielded r²=0.9995. The method achieved an LOD of 0.1 µg/mL and LOQ of 0.3 µg/mL. Method A produced accurate results for non-water-resistant sunscreens but underestimated zinc in water-resistant samples. Incorporating acetonitrile in Method B restored zinc recovery to 96–101% across all tested sunscreens. Precision RSD was <1%, and robustness tests (±10% flow rate, temperature, eluent strength) showed minimal impact on retention time, peak shape, and area.

Benefits and Practical Applications

• Automates zinc oxide analysis, reducing manual effort and variability
• Applicable to diverse sunscreen formulations including water-resistant types
• Compliant with USP compendial standards, supporting monograph modernization
• Provides high sensitivity and specificity for quality control laboratories

Future Trends and Potential Applications

• Coupling IC with mass spectrometry for multi-element profiling in personal care products
• Development of faster UHPLC-based IC separations for increased throughput
• Expansion to characterizing other metal-based UV filters in cosmetics
• Integration of automated inline sample preparation for complex formulations

Conclusion

The optimized IC method with enhanced sample preparation reliably quantifies zinc oxide in sunscreen formulations, meeting accuracy, precision, sensitivity, and robustness requirements. Method B ensures complete extraction from water-resistant products, making the protocol suitable for inclusion in USP monographs and routine QC testing.

References

1. FDA Sunscreen Drug Products for Over-the-Counter Human Use; 63 FR 56584, 1998.
2. 21 CFR 352.10 Sunscreen Drug Products; 64 FR 27687, 1999.
3. USP General Chapter <591> Zinc Determination, Pharmacopeial Forum 42(6), 2016.
4. USP General Chapter <1225> Validation of Compendial Methods, USP40 NF35, 2017.
5. USP General Chapter <621> Chromatography, USP40 NF35, 2017.